Semiconductor light-emitting element and method for manufacturing semiconductor light-emitting element

ABSTRACT

A semiconductor light-emitting element includes: an n-type clad layer of an n-type AlGaN-based semiconductor material; an active layer including a planarizing layer of an AlGaN-based semiconductor material provided on the n-type clad layer, a barrier layer of an AlGaN-based semiconductor material provided on the planarizing layer, and a well layer of an AlGaN-based semiconductor material provided on the barrier layer; and a p-type semiconductor layer provided on the active layer. The active layer emits deep ultraviolet light having a wavelength of 360 nm or shorter, and a ground level of a conduction band of the planarizing layer is lower than a ground level of a conduction band of the barrier layer and higher than a ground level of a conduction band of the well layer.

RELATED APPLICATION

The Priority is claimed to Japanese Patent Application No. 2017-044055,filed on Mar. 8, 2017, the entire content of which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a semiconductor light-emitting elementand a method for manufacturing a semiconductor light-emitting element.

2. Description of the Related Art

Recently, efforts have been made to develop semiconductor light-emittingelements for emitting deep ultraviolet light. A light-emitting elementfor emitting deep ultraviolet light includes an aluminum gallium nitride(AlGaN) based n-type clad layer, active layer, and p-type clad layerstacked successively on a substrate. It is proposed to insert aplanarizing layer of a photonic crystal structure between the n-typeclad layer and the active layer of gallium nitride (GaN) basedlight-emitting elements having a light emission wavelength of about 400nm in order to enhance the flatness of the active layer.

To output deep ultraviolet light having a wavelength of 360 nm orshorter, it is necessary to use AlGaN having a high AlN compositionratio as the n-type clad layer. Insertion of a photonic crystalstructure between the n-type clad layer with a high AlN compositionratio and the active layer affects the efficiency of injecting electronsinto the active layer seriously and results in poorer light emissioncharacteristics.

SUMMARY OF THE INVENTION

In this background, one illustrative purpose of the present invention isto provide a technology of improving the light emission characteristicsof semiconductor light-emitting elements.

A semiconductor light-emitting element according to an embodiment of thepresent invention includes: an n-type clad layer of an n-typeAlGaN-based semiconductor material; an active layer including aplanarizing layer of an AlGaN-based semiconductor material provided onthe n-type clad layer, a barrier layer of an AlGaN-based semiconductormaterial provided on the planarizing layer, and a well layer of anAlGaN-based semiconductor material provided on the barrier layer; and ap-type semiconductor layer provided on the active layer. The activelayer emits deep ultraviolet light having a wavelength of 360 nm orshorter, and an AlN molar fraction of the planarizing layer is lowerthan that of the barrier layer, and a ground level of a conduction bandof the planarizing layer is higher than that of the well layer.

According to the embodiment, the flatness of the well layer formed by anAlGaN-based semiconductor material having a low AlN composition ratio isincreased by providing the barrier layer and the well layer on theplanarizing layer having a relatively low AlN composition ratio. Byconfiguring the ground level of the conduction band of the planarizinglayer to be higher than that of the well layer, light emission in theplanarizing layer is inhibited and light emission in the highly flatwell layer is induced. This improves the light emission characteristicsof the active layer and, in particular, reduces the full width at halfmaximum of the emission spectrum.

A thickness of the planarizing layer in a direction of stack may besmaller than that of the well layer.

An AlN molar fraction of the planarizing layer may be higher than thatof the well layer.

A difference in ground level of a conduction band between theplanarizing layer and the well layer may be equal to or larger than 2%of a light energy corresponding to a wavelength of light emitted fromthe active layer.

The planarizing layer may be a first planarizing layer, and the barrierlayer may be a first barrier layer, the active layer may further includea second planarizing layer of an AlGaN-based semiconductor materialprovided between the first planarizing layer and the first barrier layerand a second barrier layer of an AlGaN-based semiconductor materialprovided between the first planarizing layer and the second planarizinglayer. An AlN molar fraction of the second planarizing layer may belower than that of the first barrier layer and the second barrier layer,and a ground level of a conduction band of the second planarizing layermay be higher than that of the active layer.

A thickness of the second planarizing layer in the direction of stackmay be smaller than a thickness of the well layer.

An AlN molar fraction of the second planarizing layer may be higher thanan AlN molar fraction of the well layer.

The active layer may further include a further barrier layer of anAlGaN-based semiconductor material provided between the n-type cladlayer and the planarizing layer.

Another embodiment of the present invention relates to a method formanufacturing a semiconductor light-emitting element adapted to emitdeep ultraviolet light having a wavelength of 360 nm or shorter. Themethod includes: forming a planarizing layer of an AlGaN-basedsemiconductor material on an n-type clad layer of an n-type AlGaN-basedsemiconductor material; forming a barrier layer of an AlGaN-basedsemiconductor material on the planarizing layer; forming a well layer ofan AlGaN-based semiconductor material on the barrier layer; and forminga p-type semiconductor layer on the well layer. An AlN molar fraction ofthe planarizing layer is lower than that of the barrier layer, and aground level of a conduction band of the planarizing layer is higherthan that of the well layer.

According to the embodiment, the flatness of the well layer formed by anAlGaN-based semiconductor material having a low AlN composition ratio isincreased by providing the barrier layer and the well layer on theplanarizing layer having a relatively low AlN composition ratio. Byconfiguring the ground level of the conduction band of the planarizinglayer to be higher than that of the well layer, light emission in theplanarizing layer is inhibited and light emission in the highly flatwell layer is induced. This improves the light emission characteristicsof the active layer and, in particular, reduces the full width at halfmaximum of the emission spectrum.

The forming of the barrier layer includes: supplying a Group-IIImaterial and a Group-V material to grow an AlGaN-based semiconductormaterial layer; and supplying a Group-V material for a duration notshorter than 6 seconds and not longer than 30 seconds while a supply ofthe Group-III material is being stopped so as to stabilize theAlGaN-based semiconductor material layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described, by way of example only, withreference to the accompanying drawings which are meant to be exemplary,not limiting, and wherein like elements are numbered alike in severalFigures, in which:

FIG. 1 is a cross-sectional view schematically showing a configurationof a semiconductor light-emitting element according to the embodiment;

FIG. 2 schematically shows an energy band of the semiconductorlight-emitting element; and

FIG. 3 is a flowchart showing a method for manufacturing thesemiconductor light-emitting element.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described by reference to the preferredembodiments. This does not intend to limit the scope of the presentinvention, but to exemplify the invention.

A brief summary will be given before describing the invention inspecific details. The embodiment relates to an aluminum gallium nitride(AlGaN) based semiconductor light-emitting element for outputting deepultraviolet light having a wavelength of 360 nm or shorter. Thelight-emitting element is provided with an n-type clad layer on asubstrate, an active layer on the n-type clad layer, and a p-typesemiconductor layer on the active layer. The active layer includes abarrier layer made of an AlGaN-based semiconductor material and a welllayer made of an AlGaN-based semiconductor material.

In the light-emitting element having the above structure, an aluminumnitride (AlN) layer is formed on a sapphire (Al₂O₃) substrate, and ann-type clad layer, an active layer, and a p-type semiconductor layer areformed on the AlN layer. As a result, more serious lattice mismatch isarisen between the n-type clad layer or the active layer and thesubstrate than in the case of using a gallium nitride (GaN) substrate.It also easily results in a rough crystal surface on an atomic level anda poorer flatness of the crystal surface. In the case of forming anactive layer, and, in particular, a well layer, on a surface having apoor flatness, it is difficult to form a well layer of an eventhickness. The emission wavelength of the active layer is affected bythe thickness of the well layer in the quantum well structure.Therefore, uneven thickness of the well layer results in variation inthe emission wavelength and spread light emission spectral width. Thiswill result in poorer monochromaticity of the output light.

The embodiment addresses the issue by inserting a planarizing layerbetween the n-type clad layer and the well layer to increase theflatness of the crystal surface of the well layer and improve theevenness of the thickness of the well layer. In particular, thedifference in grating constant between the substrate and the well layeris moderated to form a highly flat well layer by approximating the AlNcomposition ratio of the planarizing layer to that of the well layer. Infurther accordance with the embodiment, light emission in theplanarizing layer is inhibited and degradation in the light emissioncharacteristics caused by light emission in the planarizing layer isprevented by configuring the ground level of the conduction band of theplanarizing layer to be higher than that of the well layer.

A detailed description will be given of an embodiment to practice thepresent invention with reference to the drawings. Like numerals are usedin the description to denote like elements and a duplicate descriptionis omitted as appropriate. To facilitate the understanding, the relativedimensions of the constituting elements in the drawings do notnecessarily mirror the relative dimensions in the actual light-emittingelement.

FIG. 1 is a cross-sectional view schematically showing a configurationof a semiconductor light-emitting element 10 according to theembodiment. The semiconductor light-emitting element 10 is a lightemitting diode (LED) chip configured to emit “deep ultraviolet light”having a central wavelength A of about 360 nm or shorter. To output deepultraviolet light having such a wavelength, the semiconductorlight-emitting element 10 is made of an aluminum gallium nitride (AlGaN)based semiconductor material having a band gap of about 3.4 eV orlarger. The embodiment particularly shows a case of emitting deepultraviolet light having a central wavelength A of about 240 nm-350 nm.

In this specification, the term “AlGaN-based semiconductor material”refers to a semiconductor material mainly containing aluminum nitride(AlN) and gallium nitride (GaN) and shall encompass a semiconductormaterial containing other materials such as indium nitride (InN).Therefore, “AlGaN-based semiconductor materials” as recited in thisspecification can be represented by a compositionIn_(1-x-y)Al_(x)Ga_(y)N (0≤x+y≤1, 0≤x≤1, 0≤y≤1). The AlGaN-basedsemiconductor material shall contain AlN, GaN, AlGaN, indium aluminumnitride (InAlN), indium gallium nitride (InGaN), or indium aluminumgallium nitride (InAlGaN).

Of “AlGaN-based semiconductor materials”, those materials that do notsubstantially contain AlN may be distinguished by referring to them as“GaN-based semiconductor materials”. “GaN-based semiconductor materials”mainly contain GaN or InGaN and encompass materials that additionallycontain a slight amount of AlN. Similarly, of “AlGaN-based semiconductormaterials”, those materials that do not substantially contain GaN may bedistinguished by referring to them as “AlN-based semiconductormaterials”. “AlN-based semiconductor materials” mainly contain AlN orInAlN and encompass materials that additionally contain a slight amountof GaN.

The semiconductor light-emitting element 10 includes a substrate 20, abuffer layer 22, an n-type clad layer 24, an active layer 26, anelectron block layer 28, a p-type clad layer 30, an n-side electrode 32,and a p-side electrode 34.

The substrate 20 is a substrate having translucency for the deepultraviolet light emitted by the semiconductor light-emitting element 10and is, for example, a sapphire (Al₂O₃) substrate. The substrate 20includes a first principal surface 20 a and a second principal surface20 b opposite to the first principal surface 20 a. The first principalsurface 20 a is a principal surface that is a crystal growth surface forgrowing the buffer layer 22 and the layers above. The second principalsurface 20 b is a principal surface that is a light extraction substratefor extracting the deep ultraviolet light emitted by the active layer 26outside. In one variation, the substrate 20 may be an aluminum nitride(AlN) substrate or an aluminum gallium nitride (AlGaN) substrate.

The buffer layer 22 is formed on the first principal surface 20 a of thesubstrate 20. The buffer layer 22 is a foundation layer (template layer)to form the n-type clad layer 24 and the layers above. For example, thebuffer layer 22 is an undoped AlN layer and is, specifically, an AlN(HT-AlN; High Temperature AlN) layer gown at a high temperature. Thebuffer layer 22 may include an undoped AlGaN layer formed on the AlNlayer. In one variation, the buffer layer 22 may be formed only by anundoped AlGaN layer when the substrate 20 is an AlN substrate or anAlGaN substrate. In other words, the buffer layer 22 includes at leastone of an undoped AlN layer and AlGaN layer.

The n-type clad layer 24 is formed on the buffer layer 22. The n-typeclad layer 24 is an n-type AlGaN-based semiconductor material layer. Forexample, the n-type clad layer 24 is an AlGaN layer doped with silicon(Si) as an n-type impurity. The composition ratio of the n-type cladlayer 24 is selected to transmit the deep ultraviolet light emitted bythe active layer 26. For example, the n-type clad layer 24 is formedsuch that the molar fraction of AlN is 20% or higher, and, preferably,40% or higher or 50% or higher. The n-type clad layer 24 has a band gaplarger than the wavelength of the deep ultraviolet light emitted by theactive layer 26. For example, the n-type clad layer 24 is formed to havea band gap of 4.3 eV or larger. It is preferable to form the n-type cladlayer 24 such that the molar fraction of AlN is 80% or lower, i.e., theband gap is 5.5 eV or smaller. It is more preferable to form the n-typeclad layer 24 such that the molar fraction of AlN is 70% or lower (i.e.,the band gap is 5.2 eV or smaller). The n-type clad layer 24 has athickness of about 1 μm-3 μm. For example, the n-type clad layer 24 hasa thickness of about 2 μm.

The active layer 26 is made of an undoped AlGaN-based semiconductormaterial and forms a double heterojunction structure by being sandwichedby the n-type clad layer 24 and the electron block layer 28. The activelayer 26 has a quantum well structure and includes a well layer 36 madeof an AlGaN-based semiconductor material, barrier layers 40 a, 40 b, 40c (also generically referred to as a barrier layers 40) made of anAlGaN-based semiconductor material, and planarizing layers 41, 42 madeof an AlGaN-based semiconductor material. In the illustrated example,the third barrier layer 40 c, the second planarizing layer 42, thesecond barrier layer 40 b, the first planarizing layer 41, the firstbarrier layer 40 a, and the well layer 36 are stacked successively onthe n-type clad layer 24. The detailed configuration of the active layer26 will be described later with reference to FIG. 2.

The electron block layer 28 is formed on the active layer 26. Theelectron block layer 28 is a p-type AlGaN-based semiconductor materiallayer and is formed such that the molar fraction of AlN is 40% orhigher, and, preferably, 50% or higher. The electron block layer 28 maybe formed such that the molar fraction of AlN is 80% or higher or may bemade of an AlN-based semiconductor material that does not substantiallycontain GaN. The electron block layer has a thickness of about 1 nm-10nm. For example, the electron block layer has a thickness of about 2nm-5 nm. The electron block layer 28 may not be a p-type layer and maybe an undoped semiconductor layer.

The p-type clad layer 30 is a p-type semiconductor layer formed on theelectron block layer 28. The p-type clad layer 30 is a p-typeAlGaN-based semiconductor material layer. For example, the p-type cladlayer 30 is an AlGaN layer doped with magnesium (Mg) as a p-typeimpurity. The p-type clad layer 30 has a thickness of about 300 nm-700nm. For example, the p-type clad layer 30 has a thickness of about 400nm-600 nm. The p-type clad layer 30 may be made of a p-type GaN-basedsemiconductor material that does not substantially contain AlN.

The n-side electrode 32 is formed in a partial region on the n-type cladlayer 24. The n-side electrode 32 is formed by a multilayer film inwhich titanium (Ti)/aluminum (Al)/Ti/gold (Au) are layered on the n-typeclad layer 24 successively. The p-side electrode 34 is formed on thep-type clad layer 30. The p-side electrode 34 is formed by a nickel(Ni)/gold (Au) multilayer film built on the p-type clad layer 30successively.

FIG. 2 schematically shows an energy band of the semiconductorlight-emitting element 10 and, in particular, schematically shows theground level of the conduction band near the active layer 26. Asillustrated, the ground level E₀ of the well layer 36 is the lowest, andthe ground level E₁ of the planarizing layers 41, 42 is slightly higherthan the ground level of the well layer 36 by ΔE. The ground level E3 ofthe barrier layers 40 (40 a, 40 b, 40 c) is higher than the ground levelof the well layer 36 or the planarizing layers 41, 42 and higher thanthe ground level E2 of the n-type clad layer 24. The ground level E4 ofthe electron block layer 28 is higher than the ground level of thebarrier layers 40.

The well layer 36 made of an undoped AlGaN-based semiconductor material.The well layer 36 is configured to have a band gap smaller than that ofthe barrier layers 40 and an AlN molar fraction smaller than that of thebarrier layers 40. The well layer 36 is configured to form a quantumwell structure along with the adjacent barrier layer 40 and to have aband gap of 3.4 eV or larger to output deep ultraviolet light having awavelength of 360 nm or shorter. The AlN molar fraction of the welllayer 36 depends on the wavelength of light emitted by the active layer26 and is configured to be, for example, 10% or higher, and, preferably,15% or higher. The AlN molar fraction of the well layer 36 is,specifically, about 15%, 20%, 25%, 30%, 35%, or 40%.

The barrier layers 40 (40 a, 40 b, 40 c) are made of an undopedAlGaN-based semiconductor material. The barrier layers 40 are configuredto have a band gap larger than that of the well layer 36 and theplanarizing layers 41, 42 and to have an AlN molar fraction higher thanthat of these layers. The AlN molar fraction of the barrier layers 40depends on the wavelength of light emitted by the active layer 26 and isconfigured to be 40% or higher, and, preferably, 50% or higher. The AlNmolar fraction of the barrier layers 40 may be 60% or higher and may be,specifically, about 65%, 70%, 75%, 80%, or 85%.

The planarizing layers 41, 42 are made of an undoped AlGaN-basedsemiconductor material. The planarizing layers 41, 42 are configured tohave a band gap smaller than that of the barrier layers 40 and to havean AlN molar fraction lower than that of the barrier layers 40.Therefore, the planarizing layers 41, 42 form a quantum well structurealong with the adjacent barrier layers 40, respectively. Meanwhile, theplanarizing layers 41, 42 are configured such that the ground level E₁of the quantum well is higher than the ground level of the well layer 36so as to prevent substantive light emission from being inducedrecombined carriers in the planarizing layers 41, 42.

To prevent substantive light emission in the planarizing layers 41, 42,it is necessary to configure the difference ΔE (=E1−E0) in ground levelbetween the well layer 36 and the planarizing layers 41, 42 to be equalto or larger than 2%, and, preferably, 3%, of the light energy E=hc/λcorresponding to the wavelength A of the light emitted from the welllayer 36. Meanwhile, it is preferred that the AlN composition ratio ofthe well layer 36 is similar to that of the planarizing layers 41, 42 inorder to improve the flatness of the well layer 36. It is preferred thatthe difference in AlN molar fraction between the well layer 36 and theplanarizing layers 41, 42 is 10% or less (e.g., 5% or less).

The value of the ground level E₁ of the planarizing layers 41, 42 can becontrolled by appropriately selecting the AlN molar fraction of theAlGaN-based semiconductor material and the thickness of the planarizinglayers 41, 42 in the direction of stack. For example, by configuring theAlN molar fraction of the planarizing layers 41, 42 to be higher thanthat of the well layer 36, the band gap of the planarizing layers 41, 42is increased, and the ground level E₁ of the planarizing layers 41, 42is ensured to be larger than the ground level E_(o) of the well layer36. Further, by decreasing the thickness of the planarizing layers 41,42, the height of the ground level E₁ of the planarizing layers 41, 42formed in the quantum well structure is increased.

In the case that the planarizing layers 41, 42 and the well layer 36 areconfigured to have the same thickness, the ground level E₁ of theplanarizing layers 41, 42 is configured to be higher than the groundlevel E₀ of the well layer 36 by configuring the AlN composition ratioof the planarizing layers 41, 42 to be larger than the AlN compositionratio of the well layer 36. For example, the difference ΔE in groundlevel between the well layer 36 and the planarizing layers 41, 42 isconfigured to be 2% or larger by configuring the difference in AlN molarfraction between the well layer 36 and the planarizing layers 41, 42 tobe 3% or larger.

In the case that the AlN composition ratio of the planarizing layers 41,42 is configured to be equal to that of the well layer 36, on the otherhand, the ground level E₁ of the planarizing layers 41, 42 is configuredto be larger than the ground level E₀ of the well layer 36 byconfiguring the thickness of the planarizing layers 41, 42 to be smallerthan the thickness of the well layer 36. For example, the thickness ofthe well layer 36 may be about 1.5-3 nm, and the thickness of theplanarizing layers 41, 42 may be about 0.5-1.5 nm. By providing adifference in thickness of about 20%-50% between the well layer 36 andthe planarizing layers 41, 42 in the above numerical range, lightemission in the planarizing layers 41, 42 is suitably prevented.

The ground level E₁ of the planarizing layers 41, 42 may be adjusted byconfiguring both the AlN molar fraction and the thickness of theplanarizing layers 41, 42 to be different from those of the well layer36. For example, the ground level E₁ of the planarizing layers 41, 42may be configured to be higher than that of the well layer 36 byconfiguring the AlN molar fraction of the planarizing layers 41, 42 tobe lower than that of the well layer 36 and configuring the thickness ofthe planarizing layers 41, 42 to be smaller than that of the well layer36.

The first planarizing layer 41 and the second planarizing layer 42 mayhave the same ground level E₁ or different ground levels. For example,the ground level of the first planarizing layer 41 nearer the well layer36 may be configured to be higher than that of the second planarizinglayer 42 farther from the well layer 36. Conversely, the ground level ofthe first planarizing layer 41 may be configured to be lower than thatof the second planarizing layer 42. In either case, the ground level ofeach of the first planarizing layer 41 and the second planarizing layer42 is configured to be higher than the ground level E₀ of the well layer36.

In the illustrated example, two planarizing layers 41, 42 are provided.Alternatively, one planarizing layer or three or more planarizing layersmay be provided. In the case one planarizing layer is provided, thesecond barrier layer 40 b, the first planarizing layer 41, the firstbarrier layer 40 a, and the well layer 36 are stacked successively onthe n-type clad layer 24. In the case three or more planarizing layersare provided, further planarizing layers and barrier layers are insertedbetween the n-type clad layer 24 and the third barrier layer 40 c.

A description will now be given of a method for manufacturing thesemiconductor light-emitting element 10. FIG. 3 is a flowchart showing amethod for manufacturing the semiconductor light-emitting element 10.First, the substrate 20 is prepared, and the buffer layer 22 and then-type clad layer 24 are successively formed on the first principalsurface 20 a of the substrate 20 (S10).

The substrate 20 is a sapphire (Al₂O₃) substrate and is a growthsubstrate for forming an AlGaN-based semiconductor material. Forexample, the buffer layer 22 is formed on the (0001) plane of thesapphire substrate. The buffer layer 22 includes, for example, an AlN(HT-AlN) layer gown at a high temperature and an undoped AlGaN (u-AlGaN)layer. The n-type clad layer 24 is a layer made of an AlGaN-basedsemiconductor material and can be formed by using a well-known epitaxialgrowth method such as the metalorganic vapor phase epitaxy (MOVPE)method and the molecular beam epitaxy (MBE) method.

Subsequently, the barrier layer (third barrier layer 40 c) is formed onthe n-type clad layer 24, and the planarizing layer (second planarizinglayer 42) is formed on the barrier layer (S12). The barrier layer andthe planarizing layer are layers made of an AlGaN-based semiconductormaterial and can be formed by using a well-known epitaxial growth methodsuch as the metalorganic vapor phase epitaxy (MOVPE) method and themolecular beam epitaxy (MBE) method. For example, the AlGaN-basedsemiconductor material layer can be grown by supplying, as a stock gas,trimethylaluminum (TMA; (CH₃)₃Al) and trimethylgallium (TMG; CH₃)₃Ga),which are Group-III materials, and ammonia (NH₃), which is a Group-Vmaterial.

The planarizing layer is then stabilized by supplying a Group-V materialwhile the supply of a Group-III material is being stopped (S14). Thestabilizing step is performed for a duration not shorter than 6 secondsand not longer than 30 seconds. By supplying a Group-V material whilethe supply of a Group-III material is being stopped, the crystal qualityof the surface of the planarizing layer is improved, and the even moreplanarized crystal surface is formed.

If a further planarizing layer is necessary (Y in S16), the steps of S12and S14 are then repeated. For example, a further barrier layer (secondbarrier layer 40 b) is formed on the second planarizing layer 42, and afurther planarizing layer (first planarizing layer 41) is formed on thebarrier layer, and the planarizing layer thus formed is stabilized. If afurther planarizing layer is not necessary (N in S16), a barrier layer(first barrier layer 40 a) is formed on the planarizing layer, and thewell layer 36 is formed on the barrier layer. The well layer 36 is alayer made of an AlGaN-based semiconductor material and can be formed byusing a well-known epitaxial growth method such as the metalorganicvapor phase epitaxy (MOVPE) method and the molecular beam epitaxy (MBE)method. This completes the active layer 26.

The p-type semiconductor layer is then formed on the active layer 26(S20). For example, the electron block layer 28 is formed on the activelayer 26, and the p-type clad layer 30 is then formed. The electronblock layer 28 and the p-type clad layer 30 are layers made of anAlN-based semiconductor material or an AlGaN-based semiconductormaterial and can be formed by a well-known epitaxial growth method suchas the metalorganic vapor phase epitaxy (MOVPE) method and the molecularbeam epitaxy (MBE) method.

Subsequently, a mask is formed on the p-type clad layer 30, and theactive layer 26, the electron block layer 28, and the p-type clad layer30 in the exposed region, in which the mask is not formed, are removed.The active layer 26, the electron block layer 28, and the p-type cladlayer 30 may be removed by plasma etching. The n-side electrode 32 isformed on the exposed surface 24 a of the n-type clad layer 24, and thep-side electrode 34 is formed on the p-type clad layer 30 with the maskremoved. The n-side electrode 32 and the p-side electrode 34 may beformed by a well-known method such as electron beam deposition andsputtering. This completes the semiconductor light-emitting element 10shown in FIG. 1.

A description will now be given of the advantage accomplished by theembodiment with reference to examples and comparative examples.

In example 1, the ground level E₁ of the conduction band of theplanarizing layers 41, 42 is configured to be higher than the groundlevel E_(o) of the well layer 36 by forming the two planarizing layers41, 42 and configuring the AlN molar fraction of the planarizing layers41, 42 to be higher than that of the well layer 36. In example 1, astabilizing process is not performed when the planarizing layers 41, 42are formed. In example 1, the wavelength of emitted light of 285 nm, thefull width at half maximum of the emission spectrum of 17.8 nm, and thelight output of 4.3 mW were obtained when an electric current of 100 mAis conducted.

In example 2, the ground level E₁ of the conduction band of theplanarizing layers 41, 42 is configured to be higher than the groundlevel E₀ of the well layer 36 by forming the two planarizing layers 41,42 and configuring the thickness of the planarizing layers 41, 42 to besmaller than that of the well layer 36. In example 2, a stabilizingprocess is not performed when the planarizing layers 41, 42 are formed.In example 2, the wavelength of emitted light of 285 nm, the full widthat half maximum of the emission spectrum of 15.6 nm, and the lightoutput of 4.4 mW were obtained when an electric current of 100 mA isconducted.

In example 3, as in example 2, the ground level E₁ of the conductionband of the planarizing layers 41, 42 is configured to be higher thanthe ground level E₀ of the well layer 36 by configuring the thickness ofthe two planarizing layers 41, 42 to be smaller than that of the welllayer 36. In example 3, a stabilizing process is performed for aduration of 12 seconds when the planarizing layers 41, 42 are formed. Inexample 3, the wavelength of emitted light of 285 nm, the full width athalf maximum of the emission spectrum of 13.6 nm, and the light outputof 4.5 mW were obtained when an electric current of 100 mA is conducted.

In example 4, as in example 3, the ground level E₁ of the conductionband of the planarizing layers 41, 42 is configured to be higher thanthe ground level E₀ of the well layer 36 by configuring the thickness ofthe two planarizing layers 41, 42 to be smaller than that of the welllayer 36. In example 4, a stabilizing process is performed for aduration of 24 seconds when the planarizing layers 41, 42 are formed. Inexample 4, the wavelength of emitted light of 285 nm, the full width athalf maximum of the emission spectrum of 14.2 nm, and the light outputof 4.3 mW were obtained when an electric current of 100 mA is conducted.

In comparative example 1, no planarizing layers are provided, and onebarrier layer and one well layer are formed on the n-type clad layer 24.In comparative example 1, the wavelength of emitted light of 285 nm, thefull width at half maximum of the emission spectrum of 20.2 nm, and thelight output of 4.2 mW were obtained when an electric current of 100 mAis conducted.

In example 2, as in examples 2-4, the ground level E₁ of the conductionband of the planarizing layers 41, 42 is configured to be higher thanthe ground level E₀ of the well layer 36 by configuring the thickness ofthe two planarizing layers 41, 42 to be smaller than that of the welllayer 36. In comparative example 2, a stabilizing process is performedfor a duration of 36 seconds when the planarizing layers 41, 42 areformed. In comparative example 2, the wavelength of emitted light of 285nm and the full width at half maximum of the emission spectrum of 14.6nm were obtained when an electric current of 100 mA is conducted, butthe light output did not reach 4 mW.

The results above show that the light emission characteristics of thesemiconductor light-emitting element 10 are improved by inserting theplanarizing layers 41, 42 in the active layer 26. In particular, thefull width at half maximum of the emission spectrum of the semiconductorlight-emitting element 10 is reduced by about 15%-30% to enhance themonochromaticity of the semiconductor light-emitting element 10 withoutimpairing the emission intensity of the semiconductor light-emittingelement 10. A stabilizing process of 12 seconds or 24 seconds furtherenhances the monochromaticity of the semiconductor light-emittingelement 10, but a stabilizing process of 36 seconds lowers the lightoutput of the semiconductor light-emitting element 10. It is thereforenecessary that the duration of a stabilization process be shorter than36 seconds, and, preferably, not shorter than 6 seconds and not longerthan 30 seconds.

In the embodiment, the light emission characteristics of thesemiconductor light-emitting element 10 are suitably improved byproviding the two planarizing layers 41, 42 and inserting the barrierlayer (second barrier layer 40 b) between the two planarizing layers 41,42. For improvement of the flatness of the well layer 36, it isconsidered preferable to increase the thickness of the planarizing layerunderlying the well layer 36. If the thickness of the planarizing layeris increased, however, the ground level of the conduction band of theplanarizing layer is lowered, which may result in light emission in theplanarizing layer. If the AlN composition ratio of the planarizing layeris increased to prevent light emission in the planarizing layer, on theother hand, the difference in grating constant between the planarizinglayer and the well layer 36 is increased and the improvement in flatnessdue to the insertion of the planarizing layer is hindered.

According to the embodiment, the ground level E₁ of the planarizinglayers 41, 42 is configured to be higher than the ground level E₀ of thewell layer 36 by inserting a plurality of planarizing layers 41, 42 eachhaving a small thickness instead of inserting a planarizing layer havinga large thickness. By securing a large total thickness of the pluralityof planarizing layers 41, 42, the improvement in flatness due to theinsertion of the planarizing layer is enhanced.

An attempt to increase the number of planarizing layers requires anincrease in the number of barrier layers in accordance with the numberof planarizing layers, which may result in a lower efficiency ofinjecting carriers into the well layer 36. It is therefore not preferredto increase the number of planarizing layers excessively. Our knowledgeshows that the number of planarizing layers is preferably 4 or fewer,and, more preferably, about 1-3.

Described above is an explanation based on an exemplary embodiment. Theembodiment is intended to be illustrative only and it will be understoodby those skilled in the art that various design changes are possible andvarious modifications are possible and that such modifications are alsowithin the scope of the present invention.

In the embodiment described above, the third barrier layer 40 c is shownas being provided between the n-type clad layer 24 and the secondplanarizing layer 42. In a further variation, the third barrier layer 40c may not be provided, and the second planarizing layer 42 may beprovided immediately above the n-type clad layer 24.

It should be understood that the invention is not limited to theabove-described embodiment, but may be modified into various forms onthe basis of the spirit of the invention. Additionally, themodifications are included in the scope of the invention.

What is claimed is:
 1. A semiconductor light-emitting elementcomprising: an n-type clad layer of an n-type AlGaN-based semiconductormaterial; an active layer including a planarizing layer of anAlGaN-based semiconductor material provided on the n-type clad layer, abarrier layer of an AlGaN-based semiconductor material provided on theplanarizing layer, and a well layer of an AlGaN-based semiconductormaterial provided on the barrier layer; and a p-type semiconductor layerprovided on the active layer, wherein the active layer emits deepultraviolet light having a wavelength of 360 nm or shorter, and an AlNmolar fraction of the planarizing layer is lower than that of thebarrier layer, and a ground level of a conduction band of theplanarizing layer is higher than that of the well layer.
 2. Thesemiconductor light-emitting element according to claim 1, wherein athickness of the planarizing layer in a direction of stack is smallerthan that of the well layer.
 3. The semiconductor light-emitting elementaccording to claim 1, wherein an AlN molar fraction of the planarizinglayer is higher than that of the well layer.
 4. The semiconductorlight-emitting element according to claim 1, wherein a difference inground level of a conduction band between the planarizing layer and thewell layer is equal to or larger than 2% of a light energy correspondingto a wavelength of light emitted from the active layer.
 5. Thesemiconductor light-emitting element according to claim 1, wherein theplanarizing layer is a first planarizing layer, and the barrier layer isa first barrier layer, the active layer further includes: a secondplanarizing layer of an AlGaN-based semiconductor material providedbetween the first planarizing layer and the first barrier layer; and asecond barrier layer of an AlGaN-based semiconductor material providedbetween the first planarizing layer and the second planarizing layer,and an AlN molar fraction of the second planarizing layer is lower thanthat of the first barrier layer and the second barrier layer, and aground level of a conduction band of the second planarizing layer ishigher than that of the well layer.
 6. The semiconductor light-emittingelement according to claim 5, wherein a thickness of the secondplanarizing layer in the direction of stack is smaller than a thicknessof the well layer.
 7. The semiconductor light-emitting element accordingto claim 5, wherein an AlN molar fraction of the second planarizinglayer is higher than an AlN molar fraction of the well layer.
 8. Thesemiconductor light-emitting element according to claim 1, wherein theactive layer further includes a further barrier layer of an AlGaN-basedsemiconductor material provided between the n-type clad layer and theplanarizing layer.
 9. A method for manufacturing a semiconductorlight-emitting element adapted to emit deep ultraviolet light having awavelength of 360 nm or shorter, comprising: forming a planarizing layerof an AlGaN-based semiconductor material on an n-type clad layer of ann-type AlGaN-based semiconductor material; forming a barrier layer of anAlGaN-based semiconductor material on the planarizing layer; forming awell layer of an AlGaN-based semiconductor material on the barrierlayer; and forming a p-type semiconductor layer on the well layer,wherein an AlN molar fraction of the planarizing layer is lower thanthat of the barrier layer, and a ground level of a conduction band ofthe planarizing layer is higher than that of the well layer.
 10. Themethod for manufacturing a semiconductor light-emitting elementaccording to claim 9, wherein the forming of the barrier layer includes:supplying a Group-III material and a Group-V material to grow anAlGaN-based semiconductor material layer; and supplying a Group-Vmaterial for a duration not less than 6 seconds and not more than 30seconds while a supply of the Group-III material is being stopped so asto stabilize the AlGaN-based semiconductor material layer.